The present invention relates in general to a backlight device and to a display device which uses the backlight device; and, more particularly the present invention relates to a direct backlight device, which is arranged on a back surface side of a display device, and which uses solid-light-emitting-elements (LED) represented by light emitting diodes that are capable of emitting light in three colors, consisting of red, green and blue, to irradiate a light source light to the display device.
Recently, as a backlight which is used for a large-sized liquid crystal television set, a large-sized liquid crystal monitor or the like, a cold cathode fluorescent lamp has been popularly used. The backlight device which uses a cold cathode fluorescent lamp is roughly classified into a side-edge-type backlight device, in which the cold cathode fluorescent lamp is arranged on a side of a light guide body, and a direct-type backlight device, in which a plurality of cold cathode fluorescent lamps are arranged in parallel on a back surface of a liquid crystal display panel, without providing a light guide body on the back surface of the liquid crystal display panel.
While the side-edge type backlight device has the advantage that it can have a relatively compact structure, the side-edge type backlight device has a drawback in that the light utilizing efficiency is low. On the other hand, while it has a high light utilizing efficiency, the direct backlight device has a large thickness. In place of the backlight device which uses such a cold cathode fluorescent lamp as a light source, recently, a backlight device, which uses a solid-state light emitting element (LED) represented by a light emitting diode which exhibits a high color reproducibility and a rapid responsive speed and which uses no mercury, so as to satisfy the demand for preservation of the environment, has been studied and developed by concerned companies.
Recently, a light emitting diode which is capable of emitting light in three colors consisting of red, green and blue and a light emitting diode capable of emitting white light have been developed. With respect to the light emitting diode which is used as a light source of a backlight device, since it is desirable to irradiate a white light to a display panel eventually, the use of a light emitting diode that is capable of emitting white light has been considered. However, a white light emitting diode which irradiates white light obtains the white light by converting the color emitted from the blue-light light emitting diode or the ultraviolet-light light emitting diode into white light using phosphors; and, hence, there arises a drawback in that the white color light emitting diode exhibits a poor color rendering and narrows the range of color reproducibility.
On the other hand, the light emitting diodes that are capable of emitting light of three colors, consisting of red, green and blue, are superior to the white light emitting diode with respect to the point that the three-color-light-emitting light emitting diode can obtain a wide range of color reproducibility and with respect to the point that, by changing the current quantity of the light emitting diodes that are capable of emitting light of three colors, consisting of red, green and blue, the chromaticity of the backlight can be freely changed. From this viewpoint, it is preferable to use the light emitting diodes that are capable of emitting light of three colors, consisting of red, green and blue, at this stage.
With respect to the constitution of the light-emitting-diode direct backlight device which uses light emitting diodes that are capable of emitting light of three colors, consisting of red, green and blue, as a light source, such light emitting diodes may be arranged on a back surface of the display panel, and this constitution is shown in FIG. 10, which is a developed perspective view. As shown in FIG. 10, on a back surface of the liquid crystal display panel, for example, there are a plurality of light emitting diode substrates LEDP on which light emitting diodes LEDR, LEDG, LEDB, that are capable of emitting three colors of light, consisting of red, green and blue, are arranged. Further, a reflector REF is arranged on back surfaces of the light emitting diode substrates LEDP, and side reflectors REF1, REF2, REF3, REF4 are arranged on side-surface sides of the light emitting diode substrates LEDP.
Further, an optical compensation sheet stacked body OPS is arranged above the light emitting diode substrates LEDP by way of an air layer ARL, wherein the optical compensation sheet stacked body OPS is formed by stacking a first diffusion sheet DF1, a first prism sheet PRZ1, a second prism sheet PRZ2 and a second diffusion sheet DF2 in this order from the light emitting diode substrates LEDP side. Further, between this optical compensation sheet stacked body OPS and the light emitting diode substrates LEDP, on which the light emitting diodes LED are mounted, an air layer ARL which has a relatively large layer thickness is arranged.
In the backlight device having such a constitution, lights having plural colors which are emitted from the light emitting diodes LED, which are capable of emitting light of three colors consisting of red, green and blue, which constitutes the light source, is subjected to color mixing using the air layer ARL, the reflector REF and the side reflectors REF1 to REF4; and, thereafter, the light is irradiated to the back surface of the liquid crystal display panel PNL, which constitutes the display panel.
Further, in the backlight device having such a constitution, the arrangement of the light emitting diodes that are capable of emitting light of three colors, consisting of red, green and blue, as shown in a plan view of FIG. 11, adopts a structure in which there is a light emitting diode substrate LEDP on which the light emitting diodes LEDR, LEDG, LEDG, LEDB are arranged in the order of LEDR, LEDG, LEDG, LEDB in a row, which constitutes one unit, and a plurality of these units are arranged in parallel.
Here, in this case, to obtain a white mixture of light by taking the light emitting efficiencies of the light emitting diodes LEDR, LEDG, LEDB into consideration, it is preferable to set the numbers of the respective light emitting diodes LEDR, LEDG, LEDB that are capable of emitting light of respective colors to a ratio of red(R):green(G):blue(B)=1:2:1. Further, the light emitting diodes LEDR, LEDG, LEDB which are capable of emitting light of three colors are of the side emitting type, and, hence, these light emitting diodes exhibit an irradiation pattern as shown in FIG. 12. As shown in the drawing, the optical intensity assumes a peak value when an irradiation angle θ of the light L becomes θ=approximately ±80° and an angle θ which halves the optical intensity is ±20°. In such a constitution, with the use of a light source formed of side-emitting-type light emitting diodes LED, it is possible to allow the light to be incident efficiently on the display device, and, hence, the use of side-emitting-type light emitting diodes LED is extremely effective.
Here, for example, Japanese Patent Laid-Open No. 256817/2001 (literature 1) discloses that a surface light source which uses, as a light source, light emitting diodes LEDR, LEDG, LEDB that are capable of emitting light of three colors, consisting of red, green and blue, which constitute light emitting elements in a light guide body, exhibit a poor color mixing property and cannot mix a plurality of lights into a desired light color.